Java APIs

Could someone please explain to me why the following doesn�t work:

List<? extends Number> list = new ArrayList<Integer>();

list.add( new Integer(1) );

This doesn�t compile:

cannot find symbol
symbol  : method add(java.lang.Integer)
location: interface java.util.List<capture of ? extends java.lang.Number>
                list.add( new Integer(1) );

The compiler doesn�t complain when I instantiate the List, only when I try to add something to it. Why does this happen?

Thanks for the help.

Cheers
Matthijs